1. Field of the Invention
The present invention relates to a technique for forming a thermally sprayed film on an inner face of a bore of a cylinder block. In particular, the present invention relates to a technique for preventing the thermally sprayed film from peeling.
2. Description of the Related Art
A technique is known whereby, instead of attaching a cylinder liner to a bore that passes through a cylinder block of an engine, a thermally sprayed film is formed directly on an inner face of the bore. When cylinder liners are not attached, the distance between bores can be reduced, allowing the cylinder block to be miniaturized. Further, because combustion heat generated within the bore is transmitted directly to the bore, not attaching cylinder liners allows the cooling efficiency of the engine to be improved. In order to obtain a highly smoothened face, a honing process is performed on the thermally sprayed film formed on the inner face of the bore. When the thermally sprayed film processed by honing is extremely smooth, sliding resistance between the inner face of the bore and a piston can be reduced.
When the thermally sprayed film is to be formed on the inner face of the bore, a thermal spraying gun is inserted into the bore from an opening at one end thereof. The thermal spraying gun moves, while rotating, along the axial direction of the bore.
Particles of molten metal iron or the like are thermally sprayed (spray coated) towards the inner face of the bore from a thermal spraying hole provided at the tip of the thermal spraying gun. The particles sprayed from the thermal spraying hole adhere to the inner face of the bore and form the thermally sprayed film.
Particles having a small diameter—these being included among the particles that were thermally sprayed from the thermal spraying hole of the thermal spraying gun oxidize at high temperatures and form fumes (these being metal oxide particles; for example, iron oxide). When molten metal particles and fumes are sprayed, a thermally sprayed film that contains fumes is formed on the inner face of the bore. This thermally sprayed film that contains fumes has weak adhesive strength, and when the honing process is performed thereon, a portion thereof may peel away.
A technique for performing thermal spraying process while sucking air from an opening at the other end of the bore is described in Japanese Laid Open Patent Publication (TOKKAI) 20024024. The fumes are lighter than the particles that have a larger diameter. As a result, the fumes can be sucked out of the bore by performing the thermal spraying while air is being sucked from the opening of the bore. Consequently, fewer fumes are contained in the thermally sprayed film, and the adhesive strength of the thermally sprayed film is strengthened.